Is that a flywheel for regenerative breaking? Because 400 kJ is tiny tiny tiny if it is for replacing the battery / gas tank. 400 kJ is 0.15 horse-hours.

A flywheel to actually power the car proper is going to have at least three orders of magnitude more energy stored in it, that's a horse of an entirely different color.

Yeah just regenerative braking I would think. I can't wrap my head around one capable of powering a car for miles.

Specifically, it was done to give the cars equipped an advantage in road courses. The a portion of the energy that was lost during braking going into a turn becomes available for use when the car exits the turn, allowing the car to jump up to speed faster.

In the case of Porsche's design, the rear wheels were powered off the gas engine, and the front wheels were powered off the regen braking. With the flywheel charged, the driver had 4 wheel drive for a few seconds, great for pulling out of a turn and getting back up to 'cruising' speed between turns. The key is the flywheel starts losing energy almost immediately, so its a use it or lose it problem. The benefit over battery hybrids for the same task is that a flywheel system can absorb and dump energy at a much higher rate, limited by the capabilities of the motor-generator and cycling is limited by mechanical, not chemical properties. Batteries have a finite charge/discharge rate that is generally too low for the rapid charge/discharge that would be seen in a road course application.

That said, the practical applications for flywheel storage in vehicles are limited. Flywheels don't scale real well, and while you can have 2 counter-rotating flywheels cancel out the net angular momentum effects on the vehicle, you can't cheat physics - the two individual flywheels do have angular momentum, and attempting to rotate their axis of rotation will result in torque loads on their support bearings which will have to be handled by the structure. End result is you need a very beefy structure to handle anything carrying a lot of mechanical energy that is expected to turn.

There's a huge difference between a flywheel that gives you an extra 400Kj to play with - which is an extra 100hp for 5 seconds (give or take) which degrades rapidly if not used and something that will provide enough energy to haul around a 3000lb+ car through stop and go traffic.

Not having that flywheel storing energy while your at a red light would suck if your right when you say it only holds the energy for seconds.

This also sucks.

"At the NAIAS 2011, Porsche unveiled a RSR variant of their Porsche 918 concept car which uses a flywheel-based KERS that sits beside the driver in the passenger compartment and boosts the dual electric motors driving the front wheels and the 565 BHP V8 gasoline engine driving the rear to a combined power output of 767 BHP. This system has many problems including the unbalance caused to the vehicle due to the flywheel. Porsche are currently developing an electrical storage system."

Also just so people are aware some KERS are still based on storing the energy electrically.

a toy car can hold energy in flywheel for minutes (of storage) if well made. edit: in fact given the energy involved, holding it just for few seconds would require very well cooled brakes in place of a bearing.

For the battery-replacing carbon based flywheel: there's the carbon's phase diagram http://www.mathewpeet.org/images/carbon_phase_diagram.jpg , and as per calculations I posted above for flywheel with rim speed of 2km/s, the flywheel wont vaporize if the flywheel energy is dumped into heating flywheel material, i.e. the explosion can be avoided and energy can be safely dissipated.

For the battery-replacing carbon based flywheel: there's the carbon's phase diagram http://www.mathewpeet.org/images/carbon_phase_diagram.jpg , and as per calculations I posted above for flywheel with rim speed of 2km/s, the flywheel wont vaporize if the flywheel energy is dumped into heating flywheel material, i.e. the explosion can be avoided and energy can be safely dissipated.

Have two flywheels in opposite directions of spin. This negates gyroscopic forces and you can dump the energy by bringing them into contact.

Not vaporizing when you evenly distribute the kinetic energy as heat is not the same as not having any vaporization at the point of contact where all the heat is generated quickly. You slam two flywheels together with a net rim velocity of 4 km/s, and you do it fast enough and hard enough to use it as a "failure mode" in a crash to stop them spinning, and I guarantee you will vaporize a decent amount of carbon. I wouldn't want to be nearby.

Have two flywheels in opposite directions of spin. This negates gyroscopic forces and you can dump the energy by bringing them into contact.

Good in theory, really bad in practice. The external angular momentum of the system is balanced, i.e. net momentum of zero, which would allow the vehicle to turn easily. Unfortunately, each rotor still carries all its individual momentum. As a result, any maneuvering puts significant loading on the bearings and structure between the two flywheels to maintain alignment. It that fails... well that's a problem.

Now, as to dumping the energy by bringing them into contact... the you are turning all that energy into heat, and way too damned fast. It will basically vaporize both flywheels and make a huge mess. If you are storing kilowatts of energy then any way you dump it is going to be bad.

Except those aren't actually the same thing. Explosion isn't just "rapid vaporization".

I took a little license in order to copy his quip one-line posting style, but I'm sure you know what I'm talking about.

The flywheels are confined to a vacuum chamber to reduce friction, and that vacuum chamber will turn into a pipe bomb if you slam the wheels together and start vaporizing the carbon.

Even if the flywheels weren't contained in any way, with the amount of energy we're talking about, you'll still generate enough expanding gas fast enough to get a shockwave, even in the open air. It doesn't fit as well in a one-liner, but that's the definition of an explosion. The former is just a good way to cause an explosion. Sorry for not being precise.

When solids or liquids vaporize, assuming they are perfect gases, which is not a bad first assumption here, one gets 22.4 liters for each mole of solid. That would be 12 grams of pure carbon or 183 g of steel. It wouldn't take a lot of substrate to produce high pressures.

But now we need a chemist in here to "model" this vaporization due to frictional heating. What is the composition of these composites, etc?

When solids or liquids vaporize, assuming they are perfect gases, which is not a bad first assumption here, one gets 22.4 liters for each mole of solid. That would be 12 grams of pure carbon or 183 g of steel. It wouldn't take a lot of substrate to produce high pressures.

But now we need a chemist in here to "model" this vaporization due to frictional heating. What is the composition of these composites, etc?

Here's a comparison - gun powder (taken from wiki): 6 KNO3 + C7H4O —> 3 K2CO3 + CO2 + 6 CO + 2 H2O + 2 N2 plus heat. The end result is you go from having 7 moles of solid (powder) to 3 moles of solid (the K2CO3), and 11 moles of gas. If somehow you maintained STP (i.e. neglected the heat release), this would be a huge increase in volume.

In the case of two flywheels slamming together, the energy is going to be absorbed by the wheels vaporizing, and the result will be a mix of liquid and vaporized steel. The F-1 KERS system can store ~400KJ of kinetic energy. For comparison, that's about the same as 100g of TNT and without something to absorb the energy (besides the steel) and you've got a small bomb. The F-1 KERS system also only provides ~80Hp for ~7 seconds. A 'street' application would require it to store more energy to be of significant use, so everything scales.

When solids or liquids vaporize, assuming they are perfect gases, which is not a bad first assumption here, one gets 22.4 liters for each mole of solid. That would be 12 grams of pure carbon or 183 g of steel. It wouldn't take a lot of substrate to produce high pressures.

But now we need a chemist in here to "model" this vaporization due to frictional heating. What is the composition of these composites, etc?

Here's a comparison - gun powder (taken from wiki): 6 KNO3 + C7H4O —> 3 K2CO3 + CO2 + 6 CO + 2 H2O + 2 N2 plus heat. The end result is you go from having 7 moles of solid (powder) to 3 moles of solid (the K2CO3), and 11 moles of gas. If somehow you maintained STP (i.e. neglected the heat release), this would be a huge increase in volume.

In the case of two flywheels slamming together, the energy is going to be absorbed by the wheels vaporizing, and the result will be a mix of liquid and vaporized steel. The F-1 KERS system can store ~400KJ of kinetic energy. For comparison, that's about the same as 100g of TNT and without something to absorb the energy (besides the steel) and you've got a small bomb. The F-1 KERS system also only provides ~80Hp for ~7 seconds. A 'street' application would require it to store more energy to be of significant use, so everything scales.

To put this further in perspective, say we want our flywheel to give a Chevrolet Volt a 100 mile range. That's going to require about 130MJ of power. 130MJ is the energy contained in about 21Kg of HMX (high energy density explosive) or 28Kg of TNT. Oh, and about enough energy to raise ~175Kg of steel from 0C to its melting point.

When solids or liquids vaporize, assuming they are perfect gases, which is not a bad first assumption here, one gets 22.4 liters for each mole of solid. That would be 12 grams of pure carbon or 183 g of steel. It wouldn't take a lot of substrate to produce high pressures.

But now we need a chemist in here to "model" this vaporization due to frictional heating. What is the composition of these composites, etc?

Here's a comparison - gun powder (taken from wiki): 6 KNO3 + C7H4O —> 3 K2CO3 + CO2 + 6 CO + 2 H2O + 2 N2 plus heat. The end result is you go from having 7 moles of solid (powder) to 3 moles of solid (the K2CO3), and 11 moles of gas. If somehow you maintained STP (i.e. neglected the heat release), this would be a huge increase in volume.

In the case of two flywheels slamming together, the energy is going to be absorbed by the wheels vaporizing, and the result will be a mix of liquid and vaporized steel. The F-1 KERS system can store ~400KJ of kinetic energy. For comparison, that's about the same as 100g of TNT and without something to absorb the energy (besides the steel) and you've got a small bomb. The F-1 KERS system also only provides ~80Hp for ~7 seconds. A 'street' application would require it to store more energy to be of significant use, so everything scales.

To put this further in perspective, say we want our flywheel to give a Chevrolet Volt a 100 mile range. That's going to require about 130MJ of power. 130MJ is the energy contained in about 21Kg of HMX (high energy density explosive) or 28Kg of TNT. Oh, and about enough energy to raise ~175Kg of steel from 0C to its melting point.

Well, if the flywheels were made of steel and weighed 175 kg, they would melt, not vaporize, so not explode (not a big change from 0 to 25 C with an mp of 1500). However, I still don't understand how all of that kinetic energy would get lost in friction. (I'm not saying I would want to be up close and personal with this thing during the dump, just following up with Alamout on realistic failure modes).

Well, if the flywheels were made of steel and weighed 175 kg, they would melt, not vaporize, so not explode (not a big change from 0 to 25 C with an mp of 1500). However, I still don't understand how all of that kinetic energy would get lost in friction. (I'm not saying I would want to be up close and personal with this thing during the dump, just following up with Alamout on realistic failure modes).

You'd still get vaporization. The thermal conductivity of steel is pretty high, but not nearly high enough to carry heat away from the contact point fast enough to avoid vaporization. If the flywheel was diamond maybe the conductivity would be high enough to accomplish that.

The heat doesn't just magically get distributed throughout the flywheel. That shit takes time, and you don't have time.

Well, if the flywheels were made of steel and weighed 175 kg, they would melt, not vaporize, so not explode (not a big change from 0 to 25 C with an mp of 1500). However, I still don't understand how all of that kinetic energy would get lost in friction. (I'm not saying I would want to be up close and personal with this thing during the dump, just following up with Alamout on realistic failure modes).

You'd still get vaporization. The thermal conductivity of steel is pretty high, but not nearly high enough to carry heat away from the contact point fast enough to avoid vaporization. If the flywheel was diamond maybe the conductivity would be high enough to accomplish that.

The heat doesn't just magically get distributed throughout the flywheel. That shit takes time, and you don't have time.

Depending on the material, might the condensation rate match the vaporization rate? I have more trouble envisioning a negative heat of vaporization, or at least a low one.

To put this further in perspective, say we want our flywheel to give a Chevrolet Volt a 100 mile range. That's going to require about 130MJ of power. 130MJ is the energy contained in about 21Kg of HMX (high energy density explosive) or 28Kg of TNT. Oh, and about enough energy to raise ~175Kg of steel from 0C to its melting point.

This type of calculation strikes me as sensationalist. 12 gallons of gasoline works out to several times more than that in terms of potential energy, but we don't worry about gasoline tanks exploding, because they almost never actually explode. There's no reason to blithely assume that a flywheel's typical failure mode is going to be "catastrophic explosion", it's just ridiculous.

This type of calculation strikes me as sensationalist. 12 gallons of gasoline works out to several times more than that in terms of potential energy, but we don't worry about gasoline tanks exploding...

I've bolded the bit that answers why.

In short, potential energy is just that. Potential (chemical in this case). You may have to deal with it in an accident - and if you do it's a mess, but due to design it rarely comes into play.

Kinetic energy (i.e. a flywheel) is something you must deal with. It's not optional, and doing so very rapidly and safely is not a trivial matter.

Kinetic energy (i.e. a flywheel) is something you must deal with. It's not optional, and doing so very rapidly and safely is not a trivial matter.

Only because you're assuming the catastrophic outcome occurs--yes, of course you have to deal with the kinetic energy of the flywheel, if it starts to self-destruct. But this is only a significant problem if rapid self-destruction is a likely failure more of the flywheel, and that assumes some spectacularly poor engineering on the part of the flywheel designers.

Kinetic energy (i.e. a flywheel) is something you must deal with. It's not optional, and doing so very rapidly and safely is not a trivial matter.

Only because you're assuming the catastrophic outcome occurs--yes, of course you have to deal with the kinetic energy of the flywheel, if it starts to self-destruct. But this is only a significant problem if rapid self-destruction is a likely failure more of the flywheel, and that assumes some spectacularly poor engineering on the part of the flywheel designers.

It assumes the physical properties of the materials aren't able to compensate for the forces of the crash. Engineering can only do so much when the basic physics are against you.

It assumes the physical properties of the materials aren't able to compensate for the forces of the crash. Engineering can only do so much when the basic physics are against you.

That assumption is unfounded, as far as I see it. Certainly there's some point at which you can't engineer a solution, but it's not obvious that it's within realistic operating conditions. i.e. if it takes the forces from a 200mph collision to vaporize the flywheel, then it isn't an issue in a passenger vehicle.

I don't think anyone in this thread is in a position to say it would or would not be a real problem in general use. Given that, talking about high-explosive equivalents and so on seems unnecessarily alarmist.

Rosen Motors was developing a flywheel system in the late 90's. They had a working prototype vehicle. They went bankrupt during the crash test phase.

Their system was huge, and was only for acceleration, cruising was an electric turbogenerator. You'd need something much larger to not need the electric as primary power

That said, there are vehicles that are primarily flywheel based. A few buses and trains, I'm not aware of any accidents. I suspect they're solely operating on the strength of the containment vessel though.

It assumes the physical properties of the materials aren't able to compensate for the forces of the crash. Engineering can only do so much when the basic physics are against you.

That assumption is unfounded, as far as I see it. Certainly there's some point at which you can't engineer a solution, but it's not obvious that it's within realistic operating conditions. i.e. if it takes the forces from a 200mph collision to vaporize the flywheel, then it isn't an issue in a passenger vehicle.

I don't think anyone in this thread is in a position to say it would or would not be a real problem in general use. Given that, talking about high-explosive equivalents and so on seems unnecessarily alarmist.

Well, I did my master thesis in satellite control systems, primarily working with Control Moment Gyroscopes and reaction wheels... which are both very specific applications of flywheels. Early on I ran some simple stress calculations on the loads generated by rotating at several thousand RPM. For example, a 1ft diameter flywheel at 1000RPM has ~170g load at its edge, but that formula A proportional to Omega^2*R (with a term for units), so doubling the speed quadruples the loads.

As part of my studies, I got to tour the facilities of a major spacecraft CMG/RW manufacturer. They run all of their life tests in concrete/steel reinforced bunkers because if a catastrophic failure occurred it would send pieces flying hundreds of feet and the goal is to run the units until they get indications they will fail... not all indications are subtle or provide time to react.

In order to be small and light, you need to spin the flywheel very fast to store significant kinetic energy. This in turn means you are also storing a lot of angular momentum in each wheel, so gyroscopic effects will be inherently large.

Additionally, while gasoline does have much higher energy density than a high speed flywheel, or gunpowder, or TNT, its also missing its oxidizer - and this is a self limiting reaction. The suggestion of slamming 2 flywheels together to put cancel out the energy is the equivalent to neutralizing gasoline by mixing it with the proper amount of oxygen then detonating it. Gunpowder is a much better example, as it contains all that is needed to convert the chemical energy into heat, while in the flywheel example its kinetic energy into heat.

As to the general usefulness of such a system, if the point is Kinetic Energy Recovery, the questions are:1: What percentage of kinetic energy does it re-capture?2: How long can that energy be stored?3: Does dumping that energy back into the vehicle make an appreciable difference?4: What are the negative impacts (example size & weight)?

Perhaps such a system could be useful for city cars or trucks to capture and re-use energy in stop-and-go traffic, but can it do so better than a battery hybrid? For racing, I can see why a flywheel would have the advantage - its easier to build a system that can accept and dump huge currents without destroying itself - hence you can brake and accelerate at higher levels for shorter periods of time... not really an issue with road cars (well, except for people who drive like idiots, but I digress). The flip side is that flywheels don't scale real well, have gyroscopic issues, and constantly bleed energy to the bearings.

It assumes the physical properties of the materials aren't able to compensate for the forces of the crash. Engineering can only do so much when the basic physics are against you.

That assumption is unfounded, as far as I see it. Certainly there's some point at which you can't engineer a solution, but it's not obvious that it's within realistic operating conditions. i.e. if it takes the forces from a 200mph collision to vaporize the flywheel, then it isn't an issue in a passenger vehicle.

I don't think anyone in this thread is in a position to say it would or would not be a real problem in general use. Given that, talking about high-explosive equivalents and so on seems unnecessarily alarmist.

The talk of high-explosive equivalents was in relation to dumping the flywheel's energy in an emergency situation, such as a crash. It's not the crash that vaporizes the flywheel, but the energy contained in the flywheel. The reason emergency dumping came up is because a fully energized flywheel is inherently dangerous in an uncontrolled situation, like a crash. While you could propose that a flywheel be contained in a housing that would withstand the flywheel's kinetic energy if said flywheel were to no longer be suspended properly on its bearings, that housing isn't going to be feasible as part of a car. See longhornchris04's comments on flywheel testing bunkers.

Once we're at the point of not being able to build a strong enough housing to contain the instantaneous release of the flywheel's kinetic energy, we are left with two choices. We can either ignore it, and let the flywheel disintegrate in cases of bearing failure or housing deformation, or we look at ways to dump the energy quickly. Contrary to what you say, neither of those options takes great expertise to consider.

The first case is analogous to stabbing a bench grinder's wheel with a chisel, but at much higher energies (much higher RPMs, significantly higher rotating mass). If you stab a bench grinder's wheel with a chisel, the chisel is ripped out of your hands and turned into a dangerous projectile. Now imagine that with multiple chisels and a grinding wheel that shatters when you stab it. You're going to end up with something akin to a fragmentation grenade, as the kinetic energy of the flywheel is distributed among the fragments of the flywheel and its housing.

The second case isn't any better. We need to convert all that kinetic energy into another, less dangerous form. Since we're considering an emergency case, and wanting to avoid the first scenario, we don't have many alternate forms of energy. We can't get work out of that energy by doing something like accelerating the vehicle, for two reasons. First, we can't assume (in a crash) that enough of the drivetrain is still intact to function. Second, the result of your 1400Kg car getting into a crash being that it accelerates to 1,000Kmph (yes, one thousand) is not good. We can't convert that energy into stored chemical energy like a battery, because we don't have enough batteries to store that charge (if we had 260Kg worth of batteries, why do we have a flywheel too?), and we can't pump that much power into those batteries (if we had them) that quickly (you'd want to do it in seconds or less). So we're left with dumping that energy into heat. This is also something we can't do quickly enough. This is enough energy to melt 175Kg of steel. Having a puddle of steel that big as a result of an accident is something we really want to avoid. We clearly aren't carrying around enough heatsinks to dissipate that heat into the atmosphere safely. If we assumed our 1400Kg car was made of ice, we could melt it and turn it into a big puddle of warm water, but cars aren't made of ice. If we try to turn that kinetic energy into heat via frictional braking (where the discussion of explosions came from), we run into thermal conductivity problems, which results in part of the flywheel and braking system turning into hot, high pressure gas before all the kinetic energy is converted. This results in overpressure in the flywheel housing (since we aren't assuming that the flywheel is in a reinforced bunker) to the point it fails in a spectacular manner.

tl;dr: Flywheels store energy as kinetic energy, i.e. now energy. There isn't a safe and physically feasible manner to either convert that into potential energy or disperse it as thermal energy at the speeds we'd want in an emergency situation, such as a crash.

The talk of high-explosive equivalents was in relation to dumping the flywheel's energy in an emergency situation, such as a crash. It's not the crash that vaporizes the flywheel, but the energy contained in the flywheel.

...Yes, I know that. The point is that you don't need to dump energy in every collision--as a trivial example, you wouldn't want to do in a fender bender, because it poses no danger in that situation and it would be silly to try to dump the energy rather than simply let it keep spinning. If we can build a flywheel that can withstand a typical major auto collision, then the talk about dumping the energy and causing explosions is irrelevant--you just don't do it. You need a flywheel system that can handle the stress of a collision. Obviously any system that turns into a giant bomb when it gets into an accident is unsuitable, except for Ford Pintos.

Quote:

The reason emergency dumping came up is because a fully energized flywheel is inherently dangerous in an uncontrolled situation, like a crash.

It's only inherently dangerous if it's likely to fail. That's the issue. A hypothetical ideal flywheel with super strong bearings/encasing would be completely safe, because even in the worst crash it would just sit there, still spinning. We don't have one of those, but we also aren't talking about systems that are going to explode if you go over a speed bump too fast.

To put this further in perspective, say we want our flywheel to give a Chevrolet Volt a 100 mile range. That's going to require about 130MJ of power. 130MJ is the energy contained in about 21Kg of HMX (high energy density explosive) or 28Kg of TNT. Oh, and about enough energy to raise ~175Kg of steel from 0C to its melting point.

This type of calculation strikes me as sensationalist. 12 gallons of gasoline works out to several times more than that in terms of potential energy, but we don't worry about gasoline tanks exploding, because they almost never actually explode. There's no reason to blithely assume that a flywheel's typical failure mode is going to be "catastrophic explosion", it's just ridiculous.

Gasoline has to mix with air to burn/explode. There's something limiting the burn rate, you can't explosively liberate all the energy at once in standard gas tank configuration.

A flywheel doesn't have prerequisites for releasing its energy in that way.

If it can produce a harmful shockwave or more likely shrapnel, that's a problem.

The talk of high-explosive equivalents was in relation to dumping the flywheel's energy in an emergency situation, such as a crash. It's not the crash that vaporizes the flywheel, but the energy contained in the flywheel.

...Yes, I know that. The point is that you don't need to dump energy in every collision--as a trivial example, you wouldn't want to do in a fender bender, because it poses no danger in that situation and it would be silly to try to dump the energy rather than simply let it keep spinning. If we can build a flywheel that can withstand a typical major auto collision, then the talk about dumping the energy and causing explosions is irrelevant--you just don't do it. You need a flywheel system that can handle the stress of a collision. Obviously any system that turns into a giant bomb when it gets into an accident is unsuitable, except for Ford Pintos.

Quote:

The reason emergency dumping came up is because a fully energized flywheel is inherently dangerous in an uncontrolled situation, like a crash.

It's only inherently dangerous if it's likely to fail. That's the issue. A hypothetical ideal flywheel with super strong bearings/encasing would be completely safe, because even in the worst crash it would just sit there, still spinning. We don't have one of those, but we also aren't talking about systems that are going to explode if you go over a speed bump too fast.

That's the whole point of the discussion. We do not have flywheel assemblies that can withstand major collisions. It may not even be physically possible to build them at a low enough weight to make flywheels feasible for general vehicle energy storage. The rest of the discussion was just illustrations of why we must have those super strong assemblies if we want to use flywheels for primary energy storage in vehicles, by way of showing that not having them creates bombs when something fails.

Anyway, some numbers that I hope I've worked out correctly. Going back to the 130MJ storage flywheel I mentioned earlier, if we assume it weighs 100Kg and is a cylindrical shape with a radius of 40cm (since we need to mount it with its axis horizontally in a vehicle), we need to rotate it at almost 109K RPM. If we assume that the rotor is made of carbon fiber, 130MJ is very close to the ~138.5MJ maximum we could pump into our 100Kg cylindrical rotor before it flies apart on its own. That also works out to a rotor surface speed of about 8,200Km/h. Can anyone comment on what it would take to safely house a flywheel like that?

Now, as to dumping the energy by bringing them into contact... the you are turning all that energy into heat, and way too damned fast. It will basically vaporize both flywheels and make a huge mess. If you are storing kilowatts of energy then any way you dump it is going to be bad.

When I brake from 60 mph to 0 at a junction, my brakes do not become molten slag, yet they absorb the same energy.

When I brake from 60 mph to 0 at a junction, my brakes do not become molten slag, yet they absorb the same energy.

Lets say your car is 2,000 kg, breaking from 60 mph is then ~0.7 MJ of energy dissipated, and generally over several seconds. Not even remotely comparable to a flywheel storing 100+ MJ of energy for powering the car.

I'm not sure how I got stuck defending a 130MJ flywheel. To set the record straight: I've posted a few times that flywheels seem to work great for KERS but don't look like they can scale to being the storage for a fully electric vehicle.

KERS flywheels already exist and work well in the appropriate use cases--as longhornchris said, it's best if you're decelerating and accelerating a lot, which fits the profile of race cars and buses, and I would argue also most city driving.

For a high-capacity system that would have similar properties, I'm hoping for better capacitor technology. They could be charged much faster than current batteries, allowing you to capture much more energy from braking.

To put this further in perspective, say we want our flywheel to give a Chevrolet Volt a 100 mile range. That's going to require about 130MJ of power. 130MJ is the energy contained in about 21Kg of HMX (high energy density explosive) or 28Kg of TNT. Oh, and about enough energy to raise ~175Kg of steel from 0C to its melting point.

This type of calculation strikes me as sensationalist. 12 gallons of gasoline works out to several times more than that in terms of potential energy, but we don't worry about gasoline tanks exploding, because they almost never actually explode. There's no reason to blithely assume that a flywheel's typical failure mode is going to be "catastrophic explosion", it's just ridiculous.

You got stuck defending it, because you interjected that the criticism of why a 130 MJ flywheel couldn't work were "ridiculous."